Okay, we have done one post on how to fix a low neckline, but there are many, many types of low necklines and so are many ways to fix them - and here is another idea I thought might help you as it did me.
This shirt is another one of my thrift store victims. I thought it was really cute and I could simply wear a tank top underneath it to make it modest, but that can get a little annoying because a) you are wearing another layer and b) said layer keep going up, so you have to keep pulling it down: yuck :P So what I did was take a piece of black fabric (naturally you would pick whatever color matches your shirt). It was admittedly a bit larger than I needed, but this was a scrap and you trim it down later anyways.
So, I could have just hemmed the top or finished it some other easy way, but the shirt was kind of dressy, so I wanted the top of the piece to be a bit fancier. This could be easily done with some lace, but I had some sparkly netting type stuff, so I cut a 1 1/2" strip of it.
Then I folded the edges to the middle.
And folded it in half over the top of my piece of black fabric and sewed right along the edge.
Next, you need to try on the shirt and pin the top sides of the fabric to the shirt and also pin at the bottom of the 'v.'
And sew along the edge of the two sides, so the fabric is securely attatched. Then trim away the excess fabric (it really is easier to do it this way than to have a perfectly trimmed piece) and finish the edges so they don't fray when you wash the shirt.
Believe it or not, that's it - your neckline is fixed without having to wear another shirt in only a few minutes!!